This invention relates to a method for producing thin films on a variety of substrates.
There is currently a need for a wide variety of thin film types on an equally wide variety of substrates. For example, often it is desired to protect a substrate with a durable glass-like film resistant to environmental effects. In other situations, it is desired to apply a thin film imparting certain operational characteristics to the substrate, e.g., an antireflection coating or a planarization layer in a semiconductor device, e.g., a MOS device. This variety of films requires a concomitant variety of film types, especially with regard to the porosity of the film. In addition, depending upon the application, the film must have an appropriate conductivity, dielectric strength, adherability, compatibility, etc. Of course, it is also important that the film be applicable to a wide variety of substrates, e.g., metals, glasses, ceramics, polymers, etc., by preferably simple commercial techniques such as spraying, dipping, spinning, etc. It is also desirable in any thin film coating method that the option exist for curing the thin film on the substrate at a relatively low temperature, for instance where the substrate is temperature sensitive.
The most common methods of applying thin films to metal, glass, ceramic and other substrates are chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), sputtering, and thermal oxidation. All have disadvantages, especially with regard to the low temperature preparation of dielectric films as desribed below.
CVD, PECVD, and sputtering are relatively expensive techniques (due to both equipment costs and inherently low deposition rates) which are amenable primarily to small substrates. More importantly, from a technical standpoint, CVD requires that the substrate be heated to quite high temperatures (e.g., 600.degree.-800.degree. C.) during deposition to allow diffusion and reaction of the deposited species. This has several limitations: (1) it precludes the use of non-refractory substrates such as Al; (2) it normally results in polycrystalline films in which grain boundaries serve as sinks for impurities and thus provide high diffusivity paths for ionic conductivity; and (3) the deposited film is often not fully dense despite the high deposition temperature. Additionally, CVD is restricted to particular ambients for particular films, e.g., silicon nitride is always deposited under highly reducing conditions. Therefore, it is not always possible to optimize the deposition ambient for a particular substrate or process.
PECVD reduces the required deposition temperature to, e.g., 300.degree.-400.degree. C. The deposited film, however, is typically porous and for oxide films, e.g., SiO.sub.2, careful process control must be maintained in order to avoid particle formation.
Sputtered dielectric films are difficult to prepare because the sputtered species travel at different relative velocities during deposition. Thus, films tend to become enriched in heavy elements and depleted in light elements compared to the target composition. It is, therefore, difficult to control, e.g., oxygen stoichiometry in SiO.sub.2 films.
CVD, PECVD and sputtering all produce highly conformal coatings, i.e., they tend to replicate surface morphology rather than planarize it. This is a disadvantage, e.g., in many advanced MOS designs and, therefore, B, P, SiO.sub.2 films are being developed. These films flow at 800.degree. to 1000.degree. C. to provide planarization. However, this method is obviously not a low temperature process, and there remains a need for a lower temperature method to produce planarized dielectric films.
Thermal oxidation is routinely used to prepare dielectric films. This technique has the advantage that high density and high dielectric strengths (e.g., 6-7 MV cm.sup.-1 for SiO.sub.2) can be obtained, but the inherent limitations of this technique are that high temperatures (e.g., 950.degree. C. for SiO.sub.2) are required and the choice of substrates is limited primarily to Si.
A less common method of preparing an amorphous film is based on the sol-gel process. In this process, metal alkoxides of network forming cations, e.g., Si, Al, B, Ti, P, and optionally soluble salts of modifying cations, are used as glass precursors. In alcoholic solutions catalyzed by additions of acid or base, the alkoxides are partially or completely hydrolyzed and then polymerized to form molecules of glass-like oxide networks linked by bridging oxygen atoms. This technique is readily adapted to preparation of multicomponent oxide solutions as well as single component systems.
The net reactions which describe this process are generally represented as: EQU M(OR).sub.n +xH.sub.2 O.fwdarw.M(OH).sub.x (OR).sub.n-x +x ROH (1) EQU M(OH).sub.x (OR).sub.n-x .fwdarw.MO.sub.n/2 +x/2 H.sub.2 O+(n-x) (ROH) (2)
where x in reaction 1 can be varied, e.g., from about 1-20. Generally, reaction 2 does not go to completion, i.e., colloidal particles of anhydrous oxides do not result. When the growing polymers link together to form an infinite network, the solution stiffens to a gel.
The chemistry involved in the formation of these monolithic gels is well documented in the prior art. See, e.g., (1) Brinker et al, "Sol-gel Transition in Simple Silicates", J. Non-Cryst. Solids, 48 (1982) 47-64; (2) Brinker et al, "Sol-gel Transition in Simple Silicates II", J. Non-Cryst. Solids, 63 (1984) 45-59; (3) Schaefer et al, "Characterization of Polymers and Gels by Intermediate Angle X-ray Scattering", presented at the International Union of Pure and Applied Chemists MACRO'82, Amherst, MA, July 12, 1982; (4) Pettit et al, Sol-Gel Protective Coatings for Black Chrome Solar Selective Films, SPIE Vol. 324, Optical Coatings for Energy Efficiency and Solar Applications, (pub. by the Society of Photo-Optical Instrumentation Engineers, Bellingham, WA) (1982) 176-183; (5) Brinker et al, "Relationships Between the Sol to Gel and Gel to Glass Conversions", Proceedings of the International Conference on Ultrastructure Processing of Ceramics, Glasses, and Composites, (John Wiley and Sons, N.Y.) (1984); (6) Brinker et al, "Conversion of Monolithic Gels to Glasses in a Multicomponent Silicate Glass System", J. Materials Sci., 16 (1981) 1980-1988; (7) Brinker et al, "A Comparison Between the Densification Kinetics of Colloidal and Polymeric Silica Gels", Mat. Res. Soc. Symp. Proc. Vol. 32 (1984), 25-32; all of which disclosures are entirely incorporated by reference herein. For example, much work has been done in characterizing the relationship between the properties of a monolithic, bulk gel prepared by these systems and of the properties of the solution from which such a gel is made. For instance, the relationship between solution characteristics such as pH and water content for a given solution chemical composition and the size and nature of the polymer which results in solution, and the relationship between such polymer properties and the characteristics of the finally produced gel, e.g., the degree of crosslinking, the porosity of the gel, etc., have been well studied and discussed in these references; see, e.g., references (1) and (2) above.
However, the same degree of knowledge has not heretofore been established or disclosed for thin films prepared from the mentioned sol-gel solutions. The fact that gel formation can be retarded by making the solutions sufficiently dilute, e.g., less than 10% equivalent oxides, is known. In such dilutions, more typically 2-5% equivalent oxides, the solution can be applied to various substrates by conventional processes. Such thin film preparations have been reported in Brinker et al, "Sol-Gel Derived Antireflective Coatings for Silicon", 5 (1981) 159-172, which disclosure is entirely incorporated by reference herein. Under such circumstances, the partially hydrolyzed glass-like polymers react chemically with the substrate surface, thereby achieving complete wetting. This can be represented as shown below, wherein a silica-like polymer reacts with the hydroxylated monolayer of a metal, M, to produce direct M-O-Si bonds resulting in excellent adherence. ##STR1##
The thin film is glass-like and has a certain microporosity which results during evaporation of solvent following film application. In the past, a comparison has been reported between monoliths, i.e., the bulk gels discussed above, and thin films derived from sol-gel solutions of similar oxide composition. See, e.g., Brinker et al, "Comparisons of Sol-Gel Derived Thin Films with Monoliths in Multicomponent Silicate Glass Systems", Thin Solid Films, 77 (1981) 141-148, which disclosure is entirely incorporated by reference herein. In this publication, it is concluded that, as a function of conversion temperature, most film properties display trends similar to those known for monolithic systems; however, film properties for a given curing temperature consistently reflected higher density, lower surface area and higher refractive index. The differences were not explainable. It was generally found that densification behavior of films was different from that of monoliths. There was no report of how the properties of the film could be controlled by control of the various operating conditions.
In Brinker et al, "Sol-gel Derived Antireflective Coatings", Proceedings of the Distributed Solar Collector Conference, Albuquerque, NM, Mar. 1983, 68-80, it has been disclosed that the properties of thin films deposited using the sol-gel process are affected by the time of aging of the sol-gel solution from which the films are grown. In general, it was expected that increased aging would increase the pore size of the film. However, no details were given of how the system could be controlled to predetermine the desired porosity and other characteristics of the final film, taking into account all of the necessary controlling parameters. The question still remained of how processing conditions could be reliably controlled to predetermine such important characteristics in thin films since the prior art studies in this regard relate only to macrosystems and not the microsystems inherently involved in thin films.